In 1934, Enrico Fermi proposed the following 4-fermion model
Fermi assumed that -decay involved vector currents, , but in 1956 T. D. Lee and C. N. Yang noted that no experiment had been performed to test the assumption of parity invariance, that is, the invariance of particle interactions with respect to a reflection in a mirror. They suggested that certain experimental puzzles could be explained if parity was not conserved in the weak interactions. Furthermore, Lee and Yang suggested experiments to test this hypothesis. Shortly thereafter, Chien-Shiung Wu performed the experiment (see https://en.wikipedia.org/wiki/Wu_experiment) and confirmed the violation of parity invariance. The following year, Lee and Yang won the Nobel Prize in Physics. A year later, in 1958, Robert Marshak and his student George Sudarshan developed the vector axial-vector theory (V-A) of the weak interactions, which Richard Feynman and Murray Gell-Mann developed further.
We do not know why the weak interactions violate parity conservation, in particular, why they involve both vector and axial-vector currents, where . Now that we have a better understanding of the weak interactions, however, it would be simpler to say that the weak interactions treat left and right-handed fields differently. But, the V-A jargon has stuck.
The Glashow-Salam-Weinberg (GSW) Langrangian (really a Lagrangian density) is built using the fields below for the 1st generation of leptons and quarks:
which implies . Note the absence, in this model, of the field .
The GSW model, inspired by the highly successful quantum field theory of quantum electrodynamics (QED) and the quantum field theories developed by Yang and Mills is defined by the Langrangian,
which explicitly treats left-handed and right-handed fields differently. The symbols are constants called hypercharges that are chosen to obtain agreement with observations. The other symbols are:
The quantities enclosed in the first and second pair of parentheses in the Lagrangian are matrices that act on the lepton and quark doublet fields and , respectively, while the parentheses in the remaining fermionic terms enclose matrices, that is, scalars, which act on the singlet fields , , and , respectively. However, each field is intrinsically a 4-component Lorentz object: a 4-component spinor in the case of the fermionic fields and a 4-vector in the case of the bosonic fields.
In the GSW Lagrangian, the hypercharges are assigned specific numerical values. Therefore, the Lagrangian has only two free parameters, namely, the couplings and .
The GSW Lagrangian is invariant under the group . The elements of can be represented as the matrices
represent the elements of . The elements are defined by three spacetime-dependent functions, , , , each associated with group generators , , and , respectively, while the elements are defined by a single function and a single generator .
Invariance under the group means that the following replacements leave the Lagrangian unchanged,
Notice the absence of mass terms such as and in the Lagrangian. This is necessary because mass terms violate the imposed invariance.
The model above was developed by Glashow in 1961. However, because it predicted zero mass for all bosonic fields (not just for the photon), it clearly did not accord with the absence of experimental evidence for a long-range weak nuclear force. But in 1964 a mechanism for introducing mass without violating gauge invariance was developed independently by Englert & Brout, by Higgs, and by Guralnik, Hagen & Kibble in a series of papers in that year. These papers were inspired by the work of the condensed matter theorist Phil Anderson. Much to the chagrin of the other theorists, the mechanism was referred to, by Steven Weinberg, as the Higgs mechanism and the name stuck. A more accurate name, though a bit of a mouthful, would be the Anderson-Englert-Brout-Higgs-Guralnik-Hagen-Kibble mechanism.
In 1967, Salam and Weinberg, independently, showed how the Higgs mechanism can be used to correct the mass problem of the Glashow model. In addition, Weinberg introduced gauge invariant interactions between the Higgs field and the fermion fields that induced mass terms for the latter. Thus was born the electroweak model.
It is usually argued that local gauge invariance means that the phases of the quantum fields can be set to different values at different events in spacetime and that this makes sense because there is no reason why, for example, Andromedans, ten million years from now, should adopt the same phase convention as Earthlings do now. Personally, I find this argument unconvincing. There exist, after all, other symmetries with phases that remain constant over the whole of spacetime. For example, the replacements (see https://arxiv.org/pdf/hep-ph/0410370.pdf, p. 9)
which correspond to baryon number and lepton number conservation, respectively, are global symmetries that leave the Lagrangian invariant. These global symmetries were not imposed, but appear for free and are, in that sense, accidental. My question is: why is it fine to have global symmetries where certain quantities, such as and must be the same everywhere and everywhen, that is, Andromedans and Earthlings must use the same values of these phases even though they have never met, and yet it is not fine to have the same gauge field phases everywhere? There is no good reason that I can discern. I think it is better simply to accept the discovery that local gauge invariance yields theories, which for reasons not yet clear, are spectacularly successful.
The hypercharge , electric charge (in units of the proton charge and not to be confused with the quark doublet), and the third component of the (weak) isospin are related by the Gell-Mann Nishijima relation . For the left-handed neutrino field, and , therefore, the hypercharge is . For the left-handed electron field, and and again . On the other hand, since the right-handed electron field is an singlet and, therefore, transforms trivially, that is, , and therefore . We conclude that the hypercharge is a property of the doublet or singlet.
In the quark model, up quarks have electric charge (in units of the proton charge), while down quarks have electric charge . Therefore, in order to satisfy the Gell-Mann Nishijima relation we must make the assignments , , and .
If we define and , and noting that and, therefore, , we can write
In the low energy limit (, where the W boson mass GeV), the GSW model reproduces the Fermi model for -decay provided that we set
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# load symbolic algebra module
import sympy as sm
import sympy.functions.special.tensor_functions as t
from sympy.matrices import Matrix
from sympy.physics.matrices import mgamma, msigma
from sympy import I, trigsimp, radsimp
# enable pretty printing of equations
sm.init_printing()
levi = t.LeviCivita
kron = t.KroneckerDelta
Alas, the particle physics literature is littered with differing conventions for defining the fields. In the textbook by Mark Thomson, , while in the textbook by Gordon Kane (Modern Elementary Particle Physics, 2nd Edition) these fields are defined by . And in the book by Donnelly, Formaggio, Holstein, Milner, and Surrow (Foundations of Nuclear and Particle Physics), the convention is . Here, we'll follow the convention in Thomson's book.
# make the following symbols non-commutative so that the order in which
# they are written is preserved.
gamma = sm.Symbol('\gamma^{\mu}', commutative=False)
nubarL = sm.Symbol('\overline{\\nu}_L', commutative=False)
nuL = sm.Symbol('\\nu_L', commutative=False)
ebarL = sm.Symbol('\overline{e}_L', commutative=False)
eL = sm.Symbol('e_L', commutative=False)
ebarR = sm.Symbol('\overline{e}_R', commutative=False)
eR = sm.Symbol('e_R', commutative=False)
ubarL = sm.Symbol('\overline{u}_L', commutative=False)
uL = sm.Symbol('u_L', commutative=False)
ubarR = sm.Symbol('\overline{u}_R', commutative=False)
uR = sm.Symbol('u_R', commutative=False)
dbarL = sm.Symbol('\overline{d}_L', commutative=False)
dL = sm.Symbol('d_L', commutative=False)
dbarR = sm.Symbol('\overline{d}_R', commutative=False)
dR = sm.Symbol('d_R', commutative=False)
sbarL = sm.Symbol('\overline{s}_L', commutative=False)
sL = sm.Symbol('s_L', commutative=False)
sbarR = sm.Symbol('\overline{s}_R', commutative=False)
sR = sm.Symbol('s_R', commutative=False)
cbarL = sm.Symbol('\overline{c}_L', commutative=False)
cL = sm.Symbol('c_L', commutative=False)
cbarR = sm.Symbol('\overline{c}_R', commutative=False)
cR = sm.Symbol('c_R', commutative=False)
# these do not have to be non-commutative
B, W1, W2, W3 = sm.symbols("B_\mu, W^1_\mu, W^2_\mu, W^3_\mu")
Bnu, W1nu, W2nu, W3nu = sm.symbols("B_\\nu, W^1_\\nu, W^2_\\nu, W^3_\\nu")
Wplus, Wminus = sm.symbols("W^+_\mu, W^-_\mu")
Wplusnu,Wminusnu,W3nu = sm.symbols("W^+_\\nu, W^-_\\nu, W^3_\\nu")
Z, A = sm.symbols('Z_\mu, A_\mu')
Znu, Anu = sm.symbols('Z_\\nu, A_\\nu')
nuL, eL, eR, uL, uR, dL, dR, B, W1, W2, W3
Wplus, Wminus, Z, A
g1, g2, q, thetaW = sm.symbols('g_1 g_2 q \\theta_W')
YQL, YdL, YuR, YdR = sm.symbols('Y^Q_L Y^d_L Y^u_R Y^d_R')
YL, YR, YphiL = sm.symbols('Y_L Y_R Y_L^\phi')
g1, g2, q, YL, YR, YQL, YuR, YdR, YphiL, thetaW
L = Matrix([nuL, eL])
Lbar = Matrix([nubarL, ebarL]).T
Q = Matrix([uL, dL])
Qbar = Matrix([ubarL, dbarL]).T
Lbar, L, Qbar, Q
tau1 = msigma(1)
tau2 = msigma(2)
tau3 = msigma(3)
w = tau1*W1 + tau2*W2 + tau3*W3
w
The fields have the form of charged fields, which we write as , and are identified as the fields of the charged weak bosons.
w = w.subs({W1 - I*W2: sm.sqrt(2)*Wplus,
W1 + I*W2: sm.sqrt(2)*Wminus})
w
b = B*tau1**2
b
ML = (g1/2)*YL*b + (g2/2)*w
MQ = (g1/2)*YQL*b + (g2/2)*w
ML, MQ
l = sm.expand(-gamma*Lbar*ML*L)[0] + sm.expand(-gamma*Qbar*MQ*Q)[0]
l
r = (g1/2)*YR*B*ebarR*gamma*eR
r += (g1/2)*YuR*ubarR*gamma*B*uR
r += (g1/2)*YdR*dbarR*gamma*B*dR
r = -r
r
The key idea in the GSW model, as in the original 1961 model by Glashow, is that the fields and are related to and as follows
The Glashow hypothesis is that the electromagnetic and weak nuclear forces are related and, therefore, unified in the following sense. Above a certain critical temperature, the symmetry between the two forces (called the electroweak symmetry) is restored and the two forces merge into a single electroweak force. Below the critical temperature, the electroweak symmetry is said to be spontaneously broken by the change in shape of the potential energy of the Higgs field, which we discuss below. Because of the changed shape (which, to date, is introduced by hand), the theory yields infinitely many vacuum state solutions. While this infinite ensemble of vacuum state solutions collectively respect the electroweak symmetry, the particular vacuum state into which the universe settles does not.
This feature is analogous to that of an infinite ferromagnet. Above a certain critical temperature, called the Curie temperature, the magnetic moments of the ferromagnet's domains are randomly oriented. Therefore, when averaged over the whole ferromagnet there is no net magnetization and the ferromagnet exhibits an symmetry, that is, it is invariant with respect to rotations in 3-D space. However, well below the Curie temperatue, the magnetic moments line up in the same randomly selected direction, which causes the system to lose its symmetry. Thus, the ground state of the ferromagnetic with its net magnetization does not share the symmetry of the equations that describe the system. However, collectively, the infinite ensemble of possible ground states still preserves the symmetry in the sense that a simultaneous rotation of all of the ground states in the same way doesn't change the ensemble.
We should be thankful for broken symmetry because without it the universe would be far less diverse and we would not exist.
AZv = Matrix([A, Z])
rot = Matrix([[sm.cos(thetaW), -sm.sin(thetaW)],
[sm.sin(thetaW), sm.cos(thetaW)]])
BW = rot * AZv
BW
First substitute the fields and for and , then collect (that is, group) the coefficients of the latter.
a = l + r
a = a.subs(B, BW[0])
a = a.subs(W3, BW[1])
a = a.expand().collect(A).collect(Z).collect(Wplus).collect(Wminus)
a
In the above expression there is an interaction between the electromagnetic field and the neutrino current . However, so far, we have no evidence that such an interaction exists. Therefore, guided by this fact, we need to get rid of that interaction. This can be done if we set
a = a.subs(-YL*g1*sm.cos(thetaW), g2*sm.sin(thetaW))
a
So far, so good. Now, if this theory has something to do with reality, we need to recover the known form of the electromagnetic interaction, which for electrons is , where is the electic charge of the associated particle. We can do this if we impose the conditions
or, equivalently,
The above expressions imply
a = a.subs({-YR*g1*sm.cos(thetaW): 2*g2*sm.sin(thetaW) })
a
We extract the coefficients of the bosonic fields, that is, the currents, and manipulate them into the forms in which they are typically expressed.
Wpluscoeff = a.coeff(Wplus)
Wpluscoeff = Wpluscoeff.subs(g2, q/sm.sin(thetaW)).factor(q).simplify()
Wpluscoeff
Wminuscoeff = l.coeff(Wminus)
Wminuscoeff = Wminuscoeff.subs(g2, q/sm.sin(thetaW)).factor(q).simplify()
Wminuscoeff
N, E, U, D = sm.symbols('N, E, U, D')
Acoeff = a.coeff(A).expand()
Acoeff = Acoeff.subs({ebarL*gamma*eL: E,
ubarL*gamma*uL: U,
dbarL*gamma*dL: D})
Acoeff = Acoeff.collect(E).collect(U).collect(D)
Acoeff = Acoeff.subs({E:ebarL*gamma*eL,
U: ubarL*gamma*uL,
D: dbarL*gamma*dL})
Acoeff = Acoeff.subs(g1, g2*sm.sin(thetaW)/sm.cos(thetaW))
Acoeff = Acoeff.subs({g2*sm.sin(thetaW): q})
Acoeff
One of the key manipulations is substituting the appropriate expressions for the hypercharges. We assume the validity of the relation , where and , respectively, are the electric charge in units of the proton charge and the 3rd component of the isospin. Recall that for the lepton doublet . We also showed above that for the lepton singlets. Similarly, , , where is the fermion flavor, and .
Qf, Qu, Qd, Qe, Qnu = sm.symbols('Q^f, Q^u, Q^d, Q^e, Q^\\nu')
If3, Id3, Iu3, Ie3, Inu3 = sm.symbols('I^f_3, I^d_3, I^u_3, I^e_3, I^\\nu_3')
q, Qf, Qu, Qd, Qe, Qnu, If3, Id3, Iu3, Ie3, Inu3
Implement hypercharge assignments for electromagnetic current.
Acoeff = Acoeff.subs(YuR, 2*Qu)
Acoeff = Acoeff.subs(YdR, 2*Qd)
Acoeff = Acoeff.subs(YQL, 2*(Qf - If3))
Acoeff
# up quark current
old = Acoeff.coeff(ubarL*gamma*uL)
new = old.subs({2*If3: 1, Qf: Qu})
Acoeff = Acoeff.subs({old: new})
# down quark current
old = Acoeff.coeff(dbarL*gamma*dL)
new = old.subs({2*If3:-1, Qf: Qd})
Acoeff = Acoeff.subs({old: new}).simplify()
Acoeff
From the above we see that the electromagnetic current can be written as
Do some algebraic manipulations to simplify expression. Unfortunately, collect does not work with non-commutative symbols. So try the following hack.
Zcoeff = a.coeff(Z)
Zcoeff = Zcoeff.subs({nubarL*gamma*nuL: N,
ebarL*gamma*eL: E,
ubarL*gamma*uL: U,
dbarL*gamma*dL: D})
Zcoeff = Zcoeff.collect(E).collect(N).collect(U).collect(D)
Zcoeff = Zcoeff.subs({N:nubarL*gamma*nuL,
E:ebarL*gamma*eL,
U: ubarL*gamma*uL,
D: dbarL*gamma*dL})
Zcoeff = Zcoeff.subs(g1, g2*sm.sin(thetaW)/sm.cos(thetaW))
Zcoeff
Notice that the coefficients of is equal to . Therefore, let's make those substitutions in the above.
old = Zcoeff.coeff(ubarL*gamma*uL)
new = old.subs({g2*sm.cos(thetaW)/2: g2*Iu3*sm.cos(thetaW)}).factor(g2)
Zcoeff = Zcoeff.subs({old: new})
old = Zcoeff.coeff(dbarL*gamma*dL)
new = old.subs({g2*sm.cos(thetaW)/2: -g2*Id3*sm.cos(thetaW)}).factor(g2)
Zcoeff = Zcoeff.subs({old: new})
old = Zcoeff.coeff(nubarL*gamma*nuL)
new = old.subs({g2*sm.cos(thetaW)/2: g2*Inu3*sm.cos(thetaW)}).factor(g2)
Zcoeff = Zcoeff.subs({old: new})
old = Zcoeff.coeff(ebarL*gamma*eL)
new = old.subs({g2*sm.cos(thetaW)/2: -g2*Ie3*sm.cos(thetaW)}).factor(g2)
Zcoeff = Zcoeff.subs({old: new})
Zcoeff
Implement hypercharge assignments for weak neutral current. Note the value for singlet fields is always zero.
old = Zcoeff.coeff(dbarR*gamma*dR)
new = old.subs({YdR: 2*Qd}).cancel().factor(g2)
Zcoeff = Zcoeff.subs({old: new})
old = Zcoeff.coeff(ubarR*gamma*uR)
new = old.subs({YuR: 2*Qu}).cancel().factor(g2)
Zcoeff = Zcoeff.subs({old: new})
old = Zcoeff.coeff(ebarR*gamma*eR)
new = old.subs({YR: 2*Qe}).cancel().factor(g2)
Zcoeff = Zcoeff.subs({old: new})
old = Zcoeff.coeff(nubarL*gamma*nuL)
new = old.subs({YL: 2*(Qnu - Inu3)}).simplify()
Zcoeff = Zcoeff.subs({old: new})
old = Zcoeff.coeff(dbarL*gamma*dL)
new = old.subs({YQL: 2*(Qd - Id3)}).simplify()
Zcoeff = Zcoeff.subs({old: new})
old = Zcoeff.coeff(ebarL*gamma*eL)
new = old.subs({YL: 2*(Qe - Ie3)}).simplify()
Zcoeff = Zcoeff.subs({old: new})
old = Zcoeff.coeff(ubarL*gamma*uL)
new = old.subs({YQL: 2*(Qu - Iu3)}).simplify()
Zcoeff = Zcoeff.subs({old: new})
Zcoeff
Zcoeff = Zcoeff.factor(g2).subs({g2: q / sm.sin(thetaW)})
Zcoeff
The weak neutral current can be written as
Acoeff*A
Wpluscoeff*Wplus + Wminuscoeff*Wminus
Zcoeff*Z
The GSW model incorporates quantum electrodynamics (QED), predicts weak charged and neutral current interactions and incorporates the observation that neither the electromagnetic nor the weak neutral currrent interactions change flavor, while the weak charged current interactions always do. And, of course, the model incorporates the parity violation of the weak interactions.
The Higgs Lagrangian is given by
The stability of the vacuum requires . In the very early universe, we presume that , in which case , that is, the expectation (i.e., average) value of the Higgs field in the vacuum is zero. But, at some critical temperature, , there is a phase transition in which it is presumed that changes sign thereby causing the Higgs potential to develop infinitely many degenerate vacua in each of which the Higgs field is non-zero. Above , there is a unique vacuum state and this state respects the symmetry of the Lagrangian. Below , the ensemble of vacua respects the symmetry, but, the universe cools into one of the infinitely many potential vacua, which on its own breaks the symmetry. This is an example of spontaneous symmetry breaking, which we touched upon above. The specific instance is called electroweak symmetry breaking (EWSB).
The vacuum state of the universe is one in which the Higgs field behaves like a superconductor that attenuates the propagation of the and fields, thereby rendering these fields massive, but permits the free propagation of the field. (The theory is, of course, made to do this!) Moreover, the interactions between the Higgs field and the fermions, which also are put in by hand, cause a rapid flipping between the left-handed and right-handed fermion fields, which causes energy to build up within them that we interpret as the mass of the associated field quanta, that is, particles. The results from the LHC experiment are, so far, consistent with this picture.
Since there is no right-handed neutrino the flipping mechanism cannot occur and therefore the neutrino remains massless in the GSW model. However, neutrinos are now known to have (very small) masses, but, how their masses arise remains an open question.
We need to specify the value of the hypercharge associated with the doublet . As above, the hypercharge is chosen so that the Gell-Mann Nishijima relation holds, where the third component of the weak isospin for the upper component of a doublet and for the lower component. In the GSW model, it is assumed that the vacuum expectation of is
At energies not too far from the vacuum state, we assume that the Higgs field can be approximated as follows
In the following, we shall also need the conjugate field defined by
v = sm.symbols('v')
h = sm.symbols('h')
H = Matrix([0, (v+h)/sm.sqrt(2)])
F = (g1/2)*YphiL*b + (g2/2)*w
Hc= sm.I*tau2*H
F, H, Hc
y = F*H
ybar = H.T*F
f = ybar*y
f = f[0]
f = f.subs(YphiL, 1)
f = f.subs(g1*B/2 - g2*W3/2, Z * sm.sqrt(g1**2 + g2**2)/2)
f
f = f.expand()
f = f.collect(Z**2*h).collect(Z**2*v**2)
f
The terms involving the Higgs boson field in the low-energy limit are the single and di-Higgs boson interactions, while the terms are the mass terms. For a massive vector boson field, , the mass term has the form . The term is the sum of two terms quadratic in the fields and , with the same mass, so the mass term has the form . Therefore, in the GSW model the boson mass is predicted to be , while for the boson the prediction is and, therefore, , which modulo higher order corrections is in spectacular agreement with measurements as shown below.
sw2 = 0.2397 # sin^2(theta_W)
sw = sm.sqrt(sw2)
cw = sm.sqrt(1-sw2)
print('cos(theta_W) = %8.3f' % cw)
MW = 80.385 # GeV
MZ = 91.188 # GeV
cwp = MW/MZ
print('M_W / M_Z = %8.3f\t%8.3f' % (cwp, cw / cwp))
mu, lm = sm.symbols('\mu, \lambda')
V = mu*mu*H.T*H - lm*(H.T*H)**2
V = V.expand()[0]
V
V.subs(mu**2, v**2*lm)
The GSW model predicts the existence of tri and quartic Higgs boson self-interactions and a Higgs boson mass given by , which using the intriguing ansatz implies .
The interactions between the fermions and the Higgs field are presumed to be Yukawa interactions of the form
Let us pause for a minute to consider the dimensions of the above interactions. In natural units, the Lagrangian, which recall is really a Lagrangian density, has units of since the measure over spacetime is , which has dimensions of . Since a fermion mass term looks like
and , it follows that a fermion field has dimensions . Since the Higgs boson is a scalar field, it has dimensions . Consequently, the Yukawa interactions between fermions and the Higgs boson must be a gauge invariant combination of two fermion fields with a combined dimension of and the Higgs field. Also, because the Higgs field is a doublet, it is necessary to have it interact with the Dirac conjugate of a doublet field and a singlet field.
Notice also that in order to avoid a flavor changing Yukawa interaction, we need to fudge the interaction of the up quark field with the Higgs field by using the conjugate field rather than ! By any measure, the GSW model is an amazing intellectual achievement; but it is clear there is still considerable room for improvement.
ge, me = sm.symbols('g_e, m_e')
gu, mu, gd, md = sm.symbols('g_u, m_u, g_d, m_d')
gu, mu, gd, md
ge, me, gu, mu, gd, md
Hc = sm.I*tau2*H
Qbar, Q, Hc, Hc.T
Le = -ge*(Lbar*H*eR + ebarR*H.T*L)[0]*sm.sqrt(2)
Le = Le.expand()
Le = Le.simplify().expand().collect(h*ge).collect(ge*v)
Le = Le.subs(ge*v, me)
Le = Le.subs(ge, me/v)
Le
Lq = -gd*(Qbar*H*dR + dbarR*H.T*Q)[0]*sm.sqrt(2) \
-gu*(Qbar*Hc*uR + ubarR*Hc.T*Q)[0]*sm.sqrt(2)
Lq = Lq.expand()
Lq = Lq.subs(gu*v, mu)
Lq = Lq.subs(gd*v, md)
Lq = Lq.collect(gd*h)
Lq = Lq.collect(gu*h)
Lq = Lq.collect(md)
Lq = Lq.collect(mu)
Lq = Lq.subs(gu, mu/v)
Lq = Lq.subs(gd, md/v)
Lq
We see that the Yukawa couplings, together with the particular choice of the value of the Higgs field in the vacuum state, yields two important consequences as alluded to above: the first is mass terms for the fermions and the second is interactions between the Higgs boson and fermions that are proportional to the fermion mass and inversely proportional to the vacuum expectation value . This implies that the top quark coupling to the Higgs boson is close to, and may be, unity. Why that is, is yet another mystery in a long list of them.
We conclude that the Higgs mechanism generates mass terms for the weak vector bosons as well as the fermions while preserving gauge invariance.
So far we have considered only the 1st generation of particles, of which we have discovered three so far, and we have implicitly assumed that there is no "crosstalk" between the generations. However, there are decays that can be explained if "crosstalk" exists between generations. For example, the existence of decays such as
in which the couplings differ from
by the appropriate factor in order to match the measured ratio. The
pictorial explanation of these decays, just like the decay of
positronium (an electron-positron bound system), is that the particles
comprising the
annihilate, in this case to a W boson that subsequently decays
leptonically. The observed ratio is readily explained if the W boson can
interact with quarks of differing generations, but at a reduced rate
compared with that of within generation interactions. These observations
and many others were explained by the Italian physicist Cabibbo who
suggested that the lower component of the quark doublet, that is, the
down quark field, be replaced with the combination
thetaC = sm.symbols('theta_C')
xbarL, xL = sm.symbols('\overline{d^\prime}_L, '\
'd^\prime_L', commutative=False)
Nc = xbarL * gamma * xL
Nc = Nc.subs({xL: sm.cos(thetaC)*dL + sm.sin(thetaC)*sL,
xbarL: sm.cos(thetaC)*dbarL + sm.sin(thetaC)*sbarL})
Nc = Nc.expand()
Nc
In 1970, Sheldon Glashow, John Iliopoulos and Luciano Maiani (GIM) noted that if the strange quark were the lower component of another doublet,
ybarL, yL = sm.symbols('\overline{s^\prime}_L, '\
's^\prime_L',
commutative=False)
# add the GIM terms to the Cabbibo terms
yy = ybarL * gamma * yL
yy = yy.subs({yL:-sm.sin(thetaC)*dL + sm.cos(thetaC)*sL,
ybarL:-sm.sin(thetaC)*dbarL + sm.cos(thetaC)*sbarL})
yy = yy.expand()
Nc = (Id3 - Qd * sm.sin(thetaW)**2) * Nc
Nc += (Id3 - Qd * sm.sin(thetaW)**2) * yy
# and simplify
Nc = Nc.simplify()
Nc
By introducing a 2nd doublet, the GIM mechanism neatly renders the weak neutral currents diagonal again and predicts the existence of a 4th quark, the charm quark. This prediction was confirmed in stunning fashion in 1974 with the discovery of the at Brookhaven and SLAC.
Today, it is known that mixing of the down quark fields occurs across all three generations, which mixing is described by the Cabibbo-Kobayashi-Maskawa (CKM) matrix. Unfortunately, however, no one understands why mixing across the (three) generations should exist, nor why it takes the particular form that it does, and what determines the strength of the mixing.
The pure vector boson Lagrangian is given by
From
Wplusmu, Wminusmu = sm.symbols("W^+_\mu, W^-_\mu")
Wplusnu, Wminusnu = sm.symbols("W^+_\\nu, W^-_\\nu")
Zmu, Amu = sm.symbols('Z_\mu, A_\mu')
Znu, Anu = sm.symbols('Z_\\nu, A_\\nu')
dmuWplusnu = sm.Symbol('\partial_\mu W^+_\\nu')
dnuWplusmu = sm.Symbol('\partial_\\nu W^+_\mu')
dmuWminusnu = sm.Symbol('\partial_\mu W^-_\\nu')
dnuWminusmu = sm.Symbol('\partial_\\nu W^-_\mu')
dmuZnu = sm.Symbol('\partial_\mu Z_\\nu')
dnuZmu = sm.Symbol('\partial_\\nu Z_\mu')
dmuAnu = sm.Symbol('\partial_\mu A_\\nu')
dnuAmu = sm.Symbol('\partial_\\nu A_\mu')
W1mu, W2mu, W3mu = sm.symbols('W^1_\mu W^2_\mu W^3_\mu')
W1nu, W2nu, W3nu = sm.symbols('W^1_\\nu W^2_\\nu W^3_\\nu')
Bmu, Bnu = sm.symbols('B_\mu B_\\nu')
dnuW1mu = sm.Symbol('\partial_\\nu W^1_\mu')
dnuW2mu = sm.Symbol('\partial_\\nu W^2_\mu')
dnuW3mu = sm.Symbol('\partial_\\nu W^3_\mu')
dmuW1nu = sm.Symbol('\partial_\mu W^1_\\nu')
dmuW2nu = sm.Symbol('\partial_\mu W^2_\\nu')
dmuW3nu = sm.Symbol('\partial_\mu W^3_\\nu')
dmuBnu = sm.Symbol('\partial_\mu B_\\nu')
dnuBmu = sm.Symbol('\partial_\\nu B_\mu')
Vmu = Matrix([W1mu, W2mu, W3mu])
Vnu = Matrix([W1nu, W2nu, W3nu])
dnuVmu = Matrix([dnuW1mu, dnuW2mu, dnuW3mu])
dmuVnu = Matrix([dmuW1nu, dmuW2nu, dmuW3nu])
Vmu, Vnu, Bmu, Bnu, dnuVmu, dmuVnu, dnuBmu, dmuBnu
VmuVnu = Vmu.cross(Vnu)
VmuVnu
WW = dmuVnu - dnuVmu - g2*VmuVnu
BB = dmuBnu - dnuBmu
WW, BB
VV = -sm.Rational(1, 4) * (BB*BB + (WW.T*WW)[0])
VV = VV.expand()
VV
Now substitute in , , etc.
VV = VV.subs({W1mu: (Wminusmu + Wplusmu)/sm.sqrt(2),
W2mu: (Wminusmu - Wplusmu)/sm.sqrt(2),
W3mu: sm.sin(thetaW)*Amu + sm.cos(thetaW)*Zmu,
W1nu: (Wminusnu + Wplusnu)/sm.sqrt(2),
W2nu: (Wminusnu - Wplusnu)/sm.sqrt(2),
W3nu: sm.sin(thetaW)*Anu + sm.cos(thetaW)*Znu,
dnuW1mu: (dnuWminusmu + dnuWplusmu)/sm.sqrt(2),
dnuW2mu: (dnuWminusmu - dnuWplusmu)/sm.sqrt(2),
dnuW3mu: sm.sin(thetaW)*dnuAmu + sm.cos(thetaW)*dnuZmu,
dmuW1nu: (dmuWminusnu + dmuWplusnu)/sm.sqrt(2),
dmuW3nu: sm.sin(thetaW)*dmuAnu + sm.cos(thetaW)*dmuZnu,
dmuBnu: sm.cos(thetaW)*dmuAnu - sm.sin(thetaW)*dmuZnu,
dnuBmu: sm.cos(thetaW)*dnuAmu - sm.sin(thetaW)*dnuZmu})
VV = VV.expand()
VV = trigsimp(VV.subs({g2: q / sm.sin(thetaW)}))
VV
This is a complicated expression, but as expected from the form of the vector boson Lagrangian, the theory predicts tri and quartic boson interactions.
The Lagrangian of quantum chromodynamics (QCD), for the 1st generation of particles, is given by
where and , respectively, are the up and down quark field triplets each of whose components is associated with one of three quark colors. The are the 8 Gell-Mann matrices each associated with one of the 8 gluon fields , and is the strong coupling parameter. The quantities in parentheses are matrices. The QCD Lagrangian is invariant under transformations and the numbers are the structure constants of the group.
When QCD is added to the GSW (electroweak) Lagrangian, thereby forming the Lagrangian of the Standard Model (SM), the up and down quark fields acquire a 3-color index. Therefore, in the electroweak part of the Lagrangian each quark field will now carry a color index, in which case it is necessary to sum over the color indices in that part of the Lagrangian. Note that each quark field in the QCD Lagrangian is the sum of left-handed and right-handed fields.
Since the SM is gauge invariant, in order to perform calculations, it is necessary to fix the definition of its bosonic (gauge) fields by choosing a gauge. Choosing a gauge is akin to choosing a coordinate system in that the predictions of the theory do not depend on the choice of gauge, at least in the limit of exact calculations. (In practice, calculations are done using perturbation theory. Consequently, depending on how the calculations are done there may be some residual, unphysical, gauge dependence in the predictions.) Unfortunately, choosing the gauge requires the addition of quite a bit of extra structure, involving unphysical fields called ghost fields, which makes what is otherwise an elegant structure much less so.